A hammer mill in general is a rotating machine for reducing the size of a product to a smaller desired size. Hammers, either rigid or free swinging, are attached to a shaft. These hammers rotate as the shaft rotates and the tips of the hammers define a circle within their traveled path. These hammers may be any of a variety of possible shapes, but the outside tips, away from the supporting shafts axis of rotation, do the useful work.
Particles are reduced in size by explosion due to impact with hammer faces, cutting by the edge of the hammers and attrition or rubbing action on the sides of the hammers. Product which enters the mill in a reduced screen area will develop a greater amount of impact or rubbing action with the sides of the hammers causing heat and fines or dust. Product which enters from the side of the screen will have a larger amount of rubbing contact with the first row of hammers causing fines, heat, and excessive outside row hammer wear.
Hammer mills represent a well known device for the grinding decomposition of particulate material such as grains, stone, etc. Generally, the particulate material is fed into the mill and ground by the action of the hammers against the outer screen. Schober, U.S. Pat. No. 4,146,185 discloses a refuse shredder which has a series of extended plates held together on mounting shafts with removable hammers attached to the shafts. Van Ee, U.S. Pat. No. 3,807,644 discloses a chopper pump capable of chopping suspended organic particles which includes a central orifice for directing food into the chopping element. Organic material is ground or macerated by the action of rotating vanes which pass over stationary blades and a holding plate.
Engel, U.S. Pat. No. 302,387 discloses a sugar mixing and cooling apparatus which has a center feed into which sugar is introduced to come in contact with a revolving plate. Through the action of the plate, composition is directed downward and outward. Moriya, U.S. Pat. No. 3,584,334 discloses a granulator comprising several tiers of rotary blades through which particles are dropped. Moriya uses a central orifice to introduce particles into the granulator into which they drop through a blade system onto a series of plates. The particles are then sorted and flushed from the granulator.
Barrett, U.S. Pat. No. 1,845 discloses a means for separating garlic from grain through the introduction of grain into a center fed sorter. The active element in the separating device includes a system of cylindrical channels which, by forcing the separated shaft to the outside of the system, drop the shaft off into a storage drum. Blakley et al, U.S. Reissue Pat. No. 28,677 disclose a center feed waste treatment system in which waste is pumped into a holding vat and then churned and recycled to decompose organic material present in the system.
Jadouin, U.S. Pat. No. 4,214,716 discloses a center feed pulverizer including hammer elements on a central rotor which interact with impeller elements on the outer circumference of the drum which are complementarily positioned. Pinkham, U.S. Pat. No. 3,577,998 discloses a side feed impact device which has a central turbine containing hammers or rotor arms which when rotating contact elements introduced into the apparatus. Positioned on the side walls of the apparatus are impellers or arms which are complementary fixed given the action of the hammers.
However, in operation, prior art hammer mills generally forced the product into the screen by gravity. This force must be strong enough to overcome the air pressure from the fan effect of the rotating hammers.
Further, in units where the screen is a full cylinder in shape, the product is fed from the side of the cylinder. The hammers at the feed side wear more rapidly than the other hammers, since all of the product must pass through these outside edge hammers to reach the other hammers. Product is mainly introduced at the lower quadrant of the screen. As a result, an active grinding area of a comma shape is produced in the grinding chamber, utilizing one portion of the screen more than the rest.
In units where the screen is not a full cylinder in shape, product is fed into the screen radially toward the center to the hammers. Such mills may generally comprise a tear-dropped shaped screen or semi-circular screen with product introduced into the mill through the rotating hammers at the millside. A blower may generally be required to overcome the outward blowing effect of the hammers by either pressurizing the inlet or providing vacuum on the outside of the screen to provide air flow through the screen. The product is introduced at one or the other edge of an arc of a cylindrical section in a circumferential direction providing less than a 360.degree. arc of active grinding area.
As a result, there is a need for a hammer mill which allows for even grinding of product, minimal wear of mill components and easy replacement of components when necessary, among other benefits.